The overall objective of this research proposal is to gain more precise information about the biochemical mechanisms involved in the activation of prothrombin during human blood coagulation. We plan to continue work on the isolation, purification and physicochemical characterization of human blood clotting Factors X and V and their activated forms (i.e.. Sa and Va). Following this, these preparations will be examined for 1) the existence of multiple molecular forms, 2) their stoichiometry of interaction with prothrombin and 3) their precise role in the catalytic steps leading to the initial peptide bond breaking event(s) as prothrombin converts to thrombin. In addition, certain snake venoms and a bacterial extract which mimic Factor Xa in their biological capacity to directly activate prothrombin will also be studied for comparison purposes. Detailed knowledge concerning the molecular events leading to the inception of prothrombin activation and its subsequent conversion to thrombin are of importance to our overall understanding of the hemostatic processes in human blood, their control by pharmacologic agents, and the pathogenesis of hemorrhagic and thrombotic disease.